String 匹配算法（2）---第32章

3.KMP算法（Knuth-Morris-Pratt ）

该算法的与Rabin-Karp一脉相承。也就是先排除可能不是的，在可能中逐一比较。

I.π函数的得到：

COMPUTE-PREFIX-FUNCTION(P) 1 m ← length[P] 2 π[1] ← 0 3 k ← 0 4 for q ← 2 to m 5      do while k > 0 and P[k + 1] ≠ P[q] 6             do k ← π[k] 7         if P[k + 1] = P[q] 8            then k ← k + 1 9         π[q] ← k10 return πII.伪代码算法:
KMP-MATCHER(T, P) 1 n ← length[T] 2 m ← length[P] 3 π ← COMPUTE-PREFIX-FUNCTION(P) 4 q ← 0                          ▹Number of characters matched. 5 for i ← 1 to n                 ▹Scan the text from left to right. 6      do while q > 0 and P[q + 1] ≠ T[i] 7             do q ← π[q]    ▹Next character does not match. 8         if P[q + 1] = T[i] 9            then q ← q + 1      ▹Next character matches.10         if q = m                    ▹Is all of P matched?11            then print "Pattern occurs with shift" i - m12                 q ← π[q]    ▹Look for the next match.

C语言算法实现：
void COMPUTE_PREFIX_FUNCTION(char *P, char *PI){    /*---------------------------------------------------------------*/    /* Local Variables                                               */    /*---------------------------------------------------------------*/        int m = 0;int k =0,q =0;DWORD begin,end;    /*----------------------------------------------------------------*/    /* Code Body                                                      */    /*----------------------------------------------------------------*/   begin = GetTickCount();assert(P);assert(PI);m = strlen(P);PI[0] = 0;for(q=1;q<m;q++){while((k>0 )&& (P[k]!=P[q])){k = PI[k];}if(P[k]==P[q]){k = k+1;}PI[q] = k; }end = GetTickCount();printf("the time is %d ms/n",end-begin);}/***********************************************************************[in]T string*[in]P key************************************************************************/void mmi_KMP_string_match(char* T, char* P){    /*----------------------------------------------------------------*/    /* Local Variables                                                */    /*-------------------------------------------------------------*/int m,n = 0;int k =0,q,i =0;char PI[20] = {0};DWORD begin,end;    /*----------------------------------------------------------------*/    /* Code Body                                                      */   /*----------------------------------------------------------------*/    begin = GetTickCount();n = strlen(T);m = strlen(P);COMPUTE_PREFIX_FUNCTION(P,PI);q =0;for(i =0;i<n;i++){while((q>0)&&(P[q]!=T[i])){q = PI[q];}if(P[q] ==T[i]){q =q+1;}if(q==m){printf("Pattern occurs here, %d!/n",i-m+1);q = PI[q];}}end = GetTickCount();DWORD count = end - begin;printf("the time is %d ms in KMP/n",begin);printf("the time is %d ms in KMP/n",end);printf("the time is %d ms in KMP/n",count);}